The US Census Bureau aims to group regions with populations of around 4,000 people into what they call census tracts. If you zoom in on this map, you can see that a census tract is bigger than a single block, but much smaller than a neighborhood or zip code.

It should be noted that some statistics are based on a small number of cases and should be interpreted with caution, as it may be difficult to distinguish random fluctuation in disease/injury incidence from true changes in the underlying risk for the disease/injury. Rates calculated from counts of less than 20 are particularly susceptible to this phenomenon.